Air-cooling heat exchangers are used in a variety of residential, commercial and industrial refrigeration applications where temperatures of a space are maintained below the freezing point of water (32° F.). When operating at these lower temperatures and in many environments, frost or ice will accumulate on the fins and tube surfaces of the heat exchangers. The frost or ice must be periodically removed from these surfaces in order to maintain the efficiency of the cooling system.
One common method of defrosting heat exchangers involves inserting electric resistance heating elements into vacant spaces which are adjacent to a heat exchanger fin bundle. Thereafter, these heating elements are occasionally and periodically energized to warm the fin and tube surfaces to a temperature which is sufficient to melt the accumulated frost or ice. The resulting water is then captured and removed from the space which is being refrigerated. After all the fin and tube surfaces have been freed of the accumulated frost and ice, the heating elements are deenergized, and the heat exchanger is again used to reduce the refrigerated space to the desired temperature. This periodic heating and cooling of the fin and tube surfaces to render the frost and ice free is sometimes referred to as a “defrost cycle.”
During a defrost cycle, melted frost or ice, in the form of liquid water, can sometimes make its way into the vacant tube spaces occupied by the heating element. As the heat exchanger begins to cool the refrigerated space after the defrost cycle, this liquid water conformally freezes and attaches, as ice, to the heating elements and to the sides of the vacant tube spaces in which the heating elements were placed. It should be understood that this same ice which forms around the heating element will temporarily anchor the heating element to the vacant tube spaces. Still further, and due to its coefficient of linear expansion, the metal sheath which typically encloses such heating elements will contract as the temperature of the heat exchanger drops. In the case of commercial and industrial heat exchangers, these heating elements can be as long as twenty feet or more. Consequently, the contraction which is experienced by these heating elements, when cooled, can be as much as one-half inch or more. When the heat exchanger is warmed during a subsequent defrost cycle, the same metal sheath of the heating element expands due the same coefficient of linear expansion. However, the ice that is anchoring the heating element to the vacant tube spaces does not immediately melt. Consequently, the resulting expansion of the heating element will cause it to move or creep outwardly from the heat exchanger tube bundle. Once the ice is dissipated, the heating element is left in an orientation where it is displaced outwardly relative to the heat exchanger by an amount which is equal to its linear expansion.
This movement of the heating element relative to the heat exchanger occurs, to some degree, during each defrost cycle. After repeated heating and cooling cycles, the heating element will essentially “creep” or “walk” out of the heat exchanger due to this repeated contraction and expansion. If this movement of the heating element remains unchecked, this relative movement of the heating elements may cause damage to the heating elements themselves, to the electrical wiring and circuits that feed the heating elements, or to neighboring equipment. To address this problem, a rigid mounting system was designed to restrain the heating element within the heat exchanger. It was discovered, however, that these mounting arrangements were often insufficient to counter the very strong forces resulting from the thermal expansion of the metal sheaths. More specifically, even if the chosen attachment device or method was strong enough, the repeated thermal expansion and contraction of the heating elements usually resulted in some internal damage to the heat exchanger tubes, fins, or casings.
A number of inventions have been disclosed which address the myriad of issues associated with the uneven expansion and contraction of components in heat exchanging devices, and which is caused by differences in temperatures of the component parts thereof. In U.S. Pat. No. 3,643,733 to Hall, for example, a spring is used to accommodate differences in expansion rates between tubes used to carry the different fluids in a fluid-to-fluid heat exchanger. Similar approaches have been used in cryogenic devices (U.S. Pat. No. 4,194,119 to MacKenzie) and fluid heaters (U.S. Pat. No. 5,117,482 to Hauber). None of these inventions, however, provide a solution to the problems associated with the expansion of an intermittently used heating element that is not directly involved with the normal heat exchange function.
Therefore, it has long been known that it would be desirable to provide a means of restraining electric resistance heating elements in such a way so as to accommodate limited movement of the heating elements during multiple defrost cycles while simultaneously preventing damage to the heating element and the associated heat exchanger. Further, it would be desirable to provide a means whereby the heating element could be returned to its proper position within the heat exchanger after each defrost cycle without causing damage to either the heating element itself, the heat exchanger, or associated equipment during their normal expected lifetime.
A novel mounting arrangement for electric resistance heating elements which avoids the shortcomings attendant with the prior art devices and practices utilized heretofore is the subject matter of the present application.